Hafnium electrode with inclusion used in an active medium

ABSTRACT

An object of the present invention is to provide a cathode for electric arc processes in active media which is based on hafnium and comprises additions selected from a group comprising rare earths, alkali metals and alkaline earth metals and their oxides as well as transition elements of the IV, V, VI and VIII groups of the D. I. Mendeleev Table taken separately or in a combination.

[4 1 Sept. 11, 1973 1 I-IAFNIUM ELECTRODE WITH INCLUSION USED IN AN ACTIVE MEDIUM [76] Inventors: David Grigorievich Bykhovsky;

Alexandr Yakovlevich Medvedev, both of Leningrad, U.S.S.R.

[22] Filed: Dec. 23, I969 [21] Appl. No 887,742

[] Foreign Application Priority Data Jan. 8, 1969 U.S.S.R 1297701 [52] 11.8. C1. 219/145, 75/134 V [51] Int. Cl B231: /00 [58] Field of Search 219/145; 148/24, 148/26; /134 V [56] References Cited UNITED STATES PATENTS 3,163,744 12/1964 Alonsky 219/ 2,694,763 11/1954 Muller 219/74 3,136,631 6/1964 Wlodek 75/134 V 3,505,064 4/1970 Mock 75/134 V 3,515,543 6/1970 Morton 75/134 V OTHER PUBLICATIONS l-Iackhs Chemical Dictionary, 4th Edition, Grant, McGraw-l-Iill Publication, page 415 Primary Examiner.l. V. Tru'he Assistant Examiner--George A. Montanye Attorney-Waters, Roditi, Schwartz & Nissen [57] ABSTRACT An object of the present invention is to provide a cathode for electric arc processes in active media which is based on hafnium and comprises additions selected from a group comprising rare earths, alkali metals and alkaline earth metals and their oxides as well as transition elements of the IV, V, VI and VIII groups of the D. l. Mendeleev Table taken separately or in a combination.

16 Claims, No Drawings HAFNIUM ELECTRODE WITH INCLUSION USED IN AN ACTIVE MEDIUM The present invention relates to the improvement of cathodes based on hafnium and intended for electric arc processes in active media. Active media are gaseous media which comprise components capable of reacting chemically with the cathode material at working temperatures, for example, gaseous media comprising oxygen or nitrogen.

The invention can be used for plasma treatment of materials, particularly for cutting metals and metalloids, for welding, melting and surface fusion.

The invention can also be used for heating active gases and their mixtures as well as for chemical and thermal surface treatment of materials, for example in nitriding, cementation and similar processes. In addition, the proposed cathode can be used in electric are light sources.

Known in the art is a device for plasma treatment of materials in active gases, as described in applicants copending US. Patent application Ser. No. 798,348 now US. Pat. No. 3,597,649, preferably for plasma cutting of metals, in which device the cathode is made of hafnium. The resistance of such a cathode ensures its failure-proof operation within hours, and higher.

However, if the current density is increased, for example when reducing the diameter of the stabilizing nozzle of the cutting device or in case of a considerable increase in the consumption of plasma-forming gas, the resistance of the cathode is reduced, whereas the rate of combustion increases non-linearly up to an inadmissibly high value.

The resistance of the cathode based on hafnium is also decreased under conditions of repeated short operating periods and at are current exceeding 400 amperes.

When such a cathode is employed in portable devices in which the intensity of cooling of the cathode is not sufficiently high, the resistance of the cathode is also reduced.

An object of the present invention is to eliminate the above mentioned disadvantage.

The main object of the invention is to provide such a cathode based on hafnium, which due to its chemical composition would have a higher resistance as compared to the known cathodes of the same type.

This object is accomplished due to the fact that the cathode for effecting electric arc processes in active media based on hafnium, according to the invention, comprises additions of lanthanides alkali metals, alkaline earth metals and their oxides as well as additions of transition elements of the IV, V, VI and Vi] groups of the D.l.Mendeleev Table taken separately or in a combination.

in order to increase the wear resistance of the cathode at an increased current density, it is expedient to take a composition comprising lanthanum 0.2-3.0 percent by weight, lanthanum oxide 0.1-3.0 percent by weight, rare earth metals 0.5-10.0 percent by weight, oxides of rare earth metals 0.1-3.0 percent by weight.

in order to increase the wear resistance of the cathode in case of its insufficient cooling, it is expedient to use the following additions (percent by weight): lithium 0.05 to 0.5; magnesium 0.1 to 1.0; barium 0.05 to 0.5; oxides of alkali metals and alkaline earths 0.5 to 3.0.

For increasing the wear resistance of the cathode in case of repeated short operating periods it is expedient to use the following components (percent by weight): thorium 0.1 to 3.0; niobium 1.0 to 10; tantalum 1.0 to 30.0; molybdenum 0.3 to 2.5; tungsten 0.5 to 15; rhenium 0.5 to 20.0.

The use of hafnium as a basic metal of the cathode is associated with the fact that this metal has a high melting point low activity in the yield of the thermoeleetron emission and is capable of forming a film on the working surface of the cathode during its operation in an active medium. This film features a low permeability for the active components of the medium, and this fact eliminates reactive diffusion of the components of the medium into the cathode ensuring failureproof operation of the cathode. When using oxygenous and nitrogenous mixtures or air as an active medium, on the working surface of the cathode there is formed a dense film consisting of hafnium oxides and nitrides, the protective properties of which at high temperatures exceeds the protective properties of other similar compounds, for example zirconium oxides and nitrides.

To increase the resistance of the cathode on the base of hafnium, it is provided with additions providing for formation of a film or layer on the working surface of the cathode during its operation, said film or layer containing hafnium, at least one of the components of the alloying additions and at least one of the components of the active medium. The additions can be included into the composition of the cathode both separately and in combination.

According to the invention, the above-said alloying additions may be composed of additions of lanthanum, lanthanum oxides, rare earth metals, and rare earth metal oxides which contribute to refinement of the material of the cathode, i.e. refine the boundaries of the granules from harmful impurities and this, in turn, stops the boundary diffusion of the components of the medium.

in this case the lanthanum is taken in the amount 0.2 to 3.0 percent by weight, lanthanum oxide is taken in the amount of 0.1 to 3.0 percent by weight, rare earth metals are taken in the amount of 0.5 to 10.0 percent by weight.

Due to high chemical activity of the abovementioned additions, the products of their interaction with the components of the active medium are included into the composition of the film or layer, thus increasing the forces of interatomic links therein, i.e. the chemical and mechanical strength of the film. The additions can be introduced either by means of producing limited solid solutions and chemical compounds or by means of producing mechanical mixtures or other compositions by the method of power metallurgy.

According to the invention there are also used additions of alkali metals and alkaline earth metals which due to the low intensity of the yield and high affinity with the components of the active medium. for example with oxygen, are included into the film composition, thus reducing the heat flow to the cathode.

in this case lithium is taken in the amount of 0.05 to 0.5 percent by weight, magnesium is taken in the amount of 0.1 to 1.0 percent by weight, barium is taken in the amount of 0.05 to 0.5 percent by weight, oxides of alkali metals or alkaline earth metals are taken in the amount of 0.5 to 3.0 percent by weight.

The presence of these additions of alkali metals and alkaline earth metals and their oxides in the film compounds considerably increase the emissive capacity of the cathode and this is especially important in case when the cathode is used in an electric arc light source.

The above additions are introduced into the cathode compound at a high pressure, in which case the boiling point of the additions should be higher than 2,200C, i.e. the hafnium melting temperature. The cathode on the base of hafnium with additions of alkali metals, alkaline earth metals or their oxides can also be made by the method of powder metallurgy.

The additions of transition elements ofthe IV, V, VI and VII groups of the D.I.Mendeleev Table decrease the rate of diffusion of the cathode material (hafnium with additions) through the film (or layer) formed on the working surface of the cathode during its operation and also decrease the mechanical stresses on the boundary surface separating the material of the cathode and the film (or layer), thus aiding in a better coherence between the film (or layer) and the material of the cathode. Owing to this fact the cathode built around hafnium with the above-said additions offers a higher resistance to heat shocks during repeated short operating periods.

In this case thorium is taken in the amount of 0.1 to 3.0 percent by weight, niobium is taken in the amount of 1.0 to percent by weight, tantalum is taken in the amount of 1.0 to 30 percent by weight, molybdenum is taken in the amount of 0.3 to 2.5 percent by weight, tungsten is taken in the amount of 0.5 to 15.0 percent by weight, rhenium is taken in the amount of 0.5 to 20.0 percent by weight.

The introduction of the above additions can be effected by means of producing limited solid solutions, and this relates to thorium, molybdenum, tungsten, rhenium, or by obtaining the predetermined properties in case of metals forming together with hafnium continuous solid solutions, (in case of using niobium and tantalum).

Following is a description by way of example of methods of carrying the invention into effect.

EXAMPLE 1 The typical cathodes on the base of hafnium with additions of lanthanum, lanthanum oxide, rare earth metals or their oxides have the following compositions (percent by weight):

Composition No. 1:

lanthanum 1 percent rest being iodine hafnium;

Composition No. 2:

lanthanum 1 percent, the rest being iodine hafnium;

Composition No. 3:

lanthanum oxide 5 percent, lanthanum 1 percent, the rest being hafnium;

Composition No. 4:

rare earth metals 5 percent, the rest being iodine hafnium, in which case the rare earth metals, in turn, consists of cerium 60 percent, lanthanum 38 percent and 2 percent of rare earth metals from praseodymium to lutecium inclusive.

During the tests of the above-described cathodes the current density on the cathode was increased by means of reducing the diameter of the stabilizing nozzle of the plasma torch from 5 to 1 mm at a constant current and a constant consumption of plasma-forming gas or by increasing the cathode current to a value higher than 400 amperes at a constant diameter of the stabilizing nozzle and a constant consumption of plasma-forming gas or by increasing the consumption of plasmaforming gas to a value higher than 4,000 litres per hour at a constant current through the cathode and a constant diameter of the nozzle. In all cases the resistance of the above-described cathodes has increased by a factor of 1.5 to 2 as compared with the resistance of the cathode made of pure iodine hafnium.

EXAMPLE 2 The typical cathodes on the base of hafnium with additions of alkali metals, alkaline earth metals and their oxides have the following composition:

Composition No. 1:

lithium 0.15 percent, the rest being iodine hafnium.

Composition No. 2:

magnesium 0.5 percent, the rest being iodine hafnium.

The tests of the cathodes of the above-mentioned compositions have been carried out in plasma torches with diameters of the stabilizing nozzles l,2,3,4 and 5 mm. The active plasma-forming medium was taken in the form of one of the following gas mixtures whose components are taken in the following volume percentage;

nitrogen 20 percent, the rest being argon;

oxygen 20 percent, the rest being argon;

oxygen 2 percent, the rest being nitrogen;

oxygen 0.2 percent, nitrogen 20 percent, the rest being argon.

Furthermore, technical compressed air and pure oxygen were used as an active plasma-forming medium.

Composition No. 3:

barium oxide 0.5 percent, the rest being iodine hafnium;

Composition No. 4:

calcium 0.1 percent, the rest being iodine hafnium.

The tests of the above-described cathodes have been carried out in electric arc plasma torches with magnetic stabilyzation of the arc. The active media were constituted by oxygenous plasma-forming mixtures of the following compositions (the components are taken in volume percentage).

Oxygen 0.2 percent, nitrogen 20 percent, the rest being argon;

oxygen 20 percent, the rest being argon.

In addition, the tests have been effected with an active medium consisting of technical compressed air and pure oxygen. The calorimetry of the heat flow in the above cathodes has shown that in all cases the heat flow directed to the above-said cathode is about 50 percent of the heat flow to the cathode made of pure iodine hafnium and tested under the same conditions.

A decrease of the heat flow for 50 percent allowed the life of the cathode to be increased 1.5 times as high as before.

EXAMPLE 3 The typical cathodes on the base of hafnium with additions of transition elements of the IV, V, VI and VIII, groups of the D.I.Mendeleev Table have the following compositions (percent by weight):

Composition 1:

thorium 1.5 percent, the rest being iodine hafnium.

Composition 2:

niobium 3 percent, molybdenum 0.5 percent, the rest being iodine hafnium.

Composition 3:

tantalum 15 percent, the rest being iodine hafnium.

Composition 4:

molybdenum 1 percent, the rest being iodine hafnium.

Composition 5:

tungsten 0.9 percent, the rest being iodine hafnium.

Composition 6:

rhenium 3 percent, the rest being iodine hafnium.

The tests of the above-described cathodes have been effected in electric arc plasma torches employed for plasma cutting of metals. The tests have been carried out under the conditions of cutting low-carbon steel.

One cutting operation was effected during 60 seconds, the interval between the operations was 15 seconds, the cutting length was within the range from 1,000 to 3,000 mm.

Technical compressed air was used as the active plasma-forming medium.

The tests have shown that all the above-described cathodes allow 900 to 1,100 cutting operations to be effected, while the similar cathodes of pure iodine hafnium provide for 500-600 cutting operations, i.e. the resistance of the proposed cathodes for 50-70 percent exceeds that of the cathode of pure iodine hafnium.

We claim:

1. A cathode having improved wear resistance operating in an electric arc process of repeated short operating periods in an active medium, consisting essentially of nafnium and at least one additive selected from the group consisting of 0.1 to 3.0 percent by weight of thorium, 1.0 to 10 percent by weight of niobium, 1.0 to 30.0 percent by weight of tantalum; 0.3 to 2.5 percent by weight molybdenum: 0.5 to percent by weight of tungsten and 0.5 to 20.0 percent by weight of rhenium.

2. A cathode operating in an electric arc process in an active medium, said cathode being formed ofa composition consisting essentially of hafnium and at least one addition which is a metal selected from the group consisting of rare earth metals, lithium, magnesium, barium, niobium, tantalum, molybdenum, tungsten and rhenium or a metal oxide selected from the group consisting of alkali metal oxides, alkaline earth metal oxides and rare earth metal oxides, said cathode forming on its working surface during operation thereof a film comprising hafnium, at least one of said additions and at least one component of said active medium.

3. A cathode according to claim 2 in which a rare earth metal is present in the amount of 0.2 to 10.0 percent by weight.

4. A cathode according to claim 3, in which lanthanum is present in the amount of 0.2 to 3.0 percent by weight.

5. A cathode according to claim 2 in which an oxide of a rare earth metal is present in the amount of 0.1 to 3.0 percent by weight.

6. A cathode according to claim 5, in which lanthanum oxide is present in the amount of 0.1 to 3.0 percent by weight.

7. A cathode according to claim 2, in which lithium is taken in the amount of 0.05 to 0.5 percent by weight.

8. A cathode according to claim 2, in which magnesium is taken in the amount of 0.1 to 1.0 percent by weight.

9. A cathode according to claim 2, in which barium is taken in the amount of 0.05 to 0.5 percent by weight.

10. A cathode according to claim 2 in which the alkali metal oxide or alkaline earth metal oxide is present in the amount of 0.5 to 3.0 percent by weight.

11. A cathode according to claim 2, in which thorium is present in the amount of 0.1 to 3.0 percent by weight.

12. A cathode according to claim 2, in which niobium is present in the amount of 1.0 to 10.0 percent by weight.

13. A cathode according to claim 2, in which tantalum is present in the amount of 1.0 to 30.0 percent by weight.

14. A cathode according to claim 2, in which molybdenum is present in the amount of 0.3 to 2.5 percent by weight.

115. A cathode according to claim 2, in which tungsten is present in the amount of 0.5 to 15.0 percent by weight.

116. A cathode according to claim 2, in which rhenium is present in the amount of 0.5 to 20.0 percent by 

2. A cathode operating in an electric arc process in an active medium, said cathode being formed of a composition consisting essentially of hafnium and at least one addition which is a metal selected from the group consisting of rare earth metals, lithium, magnesium, barium, niobium, tantalum, molybdenum, tungsten and rhenium or a metal oxide selected from the group consisting of alkali metal oxides, alkaline earth metal oxides and rare earth metal oxides, said cathode forming on its working surface during operation thereof a film comprising hafnium, at least one of said additions and at least one component of said active medium.
 3. A cathode according to claim 2 in which a rare earth metal is present in the amount of 0.2 to 10.0 percent by weight.
 4. A cathode according to claim 3, in which lanthanum is present in the amount of 0.2 to 3.0 percent by weight.
 5. A cathode according to claim 2 in which an oxide of a rare earth metal is present in the amount of 0.1 to 3.0 percent by weight.
 6. A cathode according to claim 5, in which lanthanum oxide is present in the amount of 0.1 to 3.0 percent by weight.
 7. A cathode according to claim 2, in which lithium is taken in the amount of 0.05 to 0.5 percent by weight.
 8. A cathode according to claim 2, in which magnesium is taken in the amount of 0.1 to 1.0 percent by weight.
 9. A cathode according to claim 2, in which barium is taken in the amount of 0.05 to 0.5 percent by weight.
 10. A cathode according to claim 2 in which the alkali metal oxide or alkaline earth metal oxide is present in the amount of 0.5 to 3.0 percent by weight.
 11. A cathode according to claim 2, in which thorium is present in the amount of 0.1 to 3.0 percent by weight.
 12. A cathode according to claim 2, in which niobium is present in the amount of 1.0 to 10.0 percent by weight.
 13. A cathode according to claim 2, in which tantalum is present in the amount of 1.0 to 30.0 percent by weight.
 14. A cathode according to claim 2, in which molybdenum is present in the amount of 0.3 to 2.5 percent by weight.
 15. A cathode according to claim 2, in which tungsten is present in the amount of 0.5 to 15.0 percent by weight.
 16. A cathode according to claim 2, in which rhenium is present in the amount of 0.5 to 20.0 percent by weight. 